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This invention relates to the apparatus and methods suitable for very low Net Positive Suction Head (NPSH) cryogenic (and other low temperature boiling liquids) pump systems, either mobile or stationary, able to operate under a wide variety of liquid supply conditions. These conditions include where the pump is above, at or below the source of liquid; whether pumping to low or medium or high pressures; start-stop against discharge pressure; and from zero to low to high NPSH at the pump""s intake. Thus it can operate under conditions including where NPSH varies during use from none, to little, to much while pumping. One example of such zero NPSH difficult pumping applications is where the pump is located above a saturated or near saturated cryogenic liquid source carried in a small tank located on a vehicle (the vibration of the vehicle/motor tending to destroy any liquid stratification or pressure building result), thus providing near zero Net Positive Suction Head (NPSH) or less at the intake of the pump""s inlet conduit, a condition under which most known cryogenic pumps cannot reliably operate, especially as the tank becomes nearly empty. Furthermore, for many reasons, it may not be desirable to vent to the atmosphere vapor from the cryogenic or liquefied gas storage system; accordingly many traditional methods/techniques utilized in the cryogenic pump industry incorporating pressure building or similar techniques to provide prime or NPSH to a pump are not appropriate. A still further problem is that many such cryogenic systems are self-refrigerating, depending upon the repetitive delivery of cold liquid cryogen to provide the system""s refrigeration needs, thus such pressure building methods are undesirable, as they add heat to the system.
One specific application where all these pumping abilities would be desirable is when using LNG (liquefied natural gas) as an on-board fuel for large trucks or buses (or other large mobile powered units), using a LNG fueled engine. Typically the LNG must be delivered by a LNG tank truck or rail car from the producing point to a bulk dispensing station having a large LNG storage vessel; from which the LNG is transferred into each truck""s on-board fuel tank. Low pressure fuel storage tanks are desired so as to minimize their weight and costs. Then, as the truck""s engine requires fuel, the LNG is vaporized and supplied to the engine at a pre-determined pressure, with the desired pressure being a function of the engine""s specific design. Some engines are designed to operate at pressures below about 200 psig while others above about 2,000 psig, and still others at an intermediate pressure. One special difficulty presented at a LNG bulk station is that it is frequently desired for safety reasons to locate the LNG storage vessel underground and thus it is very inconvenient to locate a transfer pump underneath it, as is normal practice with many cryogens when stored in aboveground vessels. Depending upon a number of individual operational factors (and vessel design), the LNG in the underground vessel can be sub-cooled and/or pressurized and the vessel nearly full, thus offering substantial NPSH to an above-ground pump (once primed); or the oppositexe2x80x94at equilibrium conditions and an almost empty vessel, thus offering no NPSH to an above-ground pump; and accordingly the pump is subject to a variety of constantly changing, but normal, operating conditions.
Almost similar type difficulties and conditions are presented on the LNG fueled truck itself in providing NG to the engine. The most favored location on the truck or bus for the on-board fuel (LNG) tank(s) is low, and it would be inconvenient and unsafe to position a pump below the tank in an attempt to provide NPSH to the pump. In addition, the almost constant movement of a truck or bus (and of consequence the LNG fuel tank) causes the LNG throughout the tank to be at near equilibrium conditions, again making the provision of NPSH difficult.
Furthermore, it is desirable to be able to utilize as fuel nearly all the LNG in the tank, thus the ability to pump from a near empty fuel tank is desirable. A special difficulty of cryogenic and liquefied gas systems is that it is desirable to conserve the refrigeration potential of the stored liquid to the greatest extent possible, so that no venting of the cryogen or liquefied gas occurs, either when fuel is being used or when the truck is at rest; accordingly any heat conductive connections to the pump should be such that the heat leak caused by the pump is minimized. A still further difficulty is the wide range of fuel (LNG) supply rates required by trucks or buses and thus pumping capabilities required as the vehicle""s engine goes from no use to idle to mid speed and to high speed in highly variable sequences on an as-needed basis. Different engines have different desired supply/injection pressures, but one current desire is to favor injection at higher pressures because of increased efficiency and reduced pollutants in the engine""s exhaust gas. While it is theoretically possible to inject the LNG into the engine while in the liquid state (as with diesel fuel), the problems of variable volumetric efficiency associated with cryogenic pumps and the variation in LNG""s density associated with its saturation pressure have made this unfeasible. Accordingly, designers have favored vaporizing the LNG after pumping it to the desired pressure and then supplying/injecting the natural gas (NG) to the engine as compressed natural gas (CNG). This typically requires a vaporizer (using the atmosphere and/or waste engine heat or other heat source) for warming the now pressurized LNG thus forming CNG, which is then stored in a small pressure vessel maintained between two pressures, the lower pressure of which is the minimum supply/injection pressure and the upper pressure of which is determined by system capabilities or other factors and delivered through a pressure regulator at the desired pressure; all controlled by a device to monitor the pressures and cause the pump to operate.
The U.S. Dept. of Energy (DOE) in a Small Business Innovation Research Program Solicitation No. DOE/ER0686 identified xe2x80x9cLiquid Natural Gas Storage for Heavy Vehiclesxe2x80x9d as a technical topic in which DOE has a R and D mission. In this Solicitation, on-board medium pressure (about 500 psig) and high pressure (about 3,000 psig) cryogenic pumps for LNG fueled vehicles were identified as specific areas where innovation was specifically desired. A related pump use is where it is desired to also be able to provide CNG or LNG at the bulk dispensing station for charging the truck""s small pressure vessel or similar uses, thus a high pressure transfer pump is needed capable of pumping from a LNG source lower than itself (underground).
While LNG in mobile applications is used as an example herein, almost every cryogenic liquid being pumped from storage under conditions wherein a reduction in pressure below the liquid""s equilibrium pressure or where the incursion of heat into the liquid, causes part of the intake liquid to vaporize would present similar difficulties. This includes cryogens which vaporize easily from heat incursion, and also liquefied gases, which while less sensitive to heat incursion, vaporize readily from a reduction in pressure.
These problems have generally been addressed by pumps characterized by the term xe2x80x9clow NPSHxe2x80x9d pumps. Included in previous low NPSH designs are U.S. Pat. No. 3,011,450 issued Dec. 5, 1961; U.S. Pat. No. 3,023,710 issued Mar. 6, 1962; U.S. Pat. No. 3,263,622 issued Aug. 2, 1966; U.S. Pat. No. 3,277,797 issued Oct. 11, 1966; and U.S. Pat. No. 6,006,525 issued Dec. 28, 1999xe2x80x94all to the present inventor. Also U.S. Pat. No. 5,188,519 issued Feb. 23, 1993 to I. S. Spulgis. In particular, these patents illustrate a type of reciprocating pumping mechanism where the intake valve is caused to open by the mechanical action of the piston rod retracting from a center opening in a hollow piston, a type of action commonly referred to as a xe2x80x9clost motionxe2x80x9d action, as the piston does not move as far as does the piston rod. This mechanical opening of the intake valve reduces one principal need for NPSH, that of causing the intake valve to open by a reduction in pressure across it. In addition, if the intake valve is located above the compression chamber, vapor in the compression chamber can escape backwards by rising through the open intake valve. These designs require that the pumping chamber be located even with or lower than the source of liquid for optimum low NPSH service.
U.S. Pat. No. 5,411,374 issued May 2, 1999 to A. Gram represents a pump design able to be located above the supply container and able to pump saturated liquid from the container""s bottom, a condition described by Gram as xe2x80x9cnegative feed pressurexe2x80x9d. The pump essentially has a double acting piston removing vapor in the pump""s inlet conduit at a rate sufficiently fast that liquid rises into the pump; as Gram states xe2x80x9cby removing vapor from liquid in an inlet conduit faster than the liquid therein can vaporize by absorbing heat. . . xe2x80x9d However, absorbing heat is but one element in the source of vapor, as equilibrium liquid almost instantaneously releases vapor (and cools itself by evaporative cooling) as its pressure is reduced. In any event, the Gram pump is essentially a pump and/or compressor, handling intermittently under the different conditions that are encountered when pumping such liquids: all vapor, or vapor and liquid mixed, or all liquid. When handling all vapor it becomes a single stage compressor, with all the limitationsxe2x80x94when compared to a single stage pumpxe2x80x94of a single stage compressor, i.e.: greatly increased power; greatly increased heat generation (heat of compression); greatly reduced capacity; and greatly reduced possible pressure differentials. When handling vapor and liquid mixed (and at low NPSH or xe2x80x9cnegative feed pressurexe2x80x9d), cavitation occurs and the pump""s volumetric efficiency (and output) become unpredictably reduced, sometimes to the extent that vapor locking and pumping failure results, especially so when operating at compression ratios of about 10 or more.
U.S. Pat. No. 5,575,626 issued Nov. 19, 1996 to Brown et al is a pump submerged from the top into a container to the bottom. However, the mechanisms represent a serious and constant heat leak and the pump requires positive NPSH to open its spring loaded inlet valve.
U.S. Pat. No. 5,787,940 issued Aug. 4, 1998 to Bonn et al is a pump submerged from the top to the bottom of a separate sump attached to the storage vessel, so that the sump can be flooded with liquid when the pump is in use or not flooded when not in use, so as to reduce the heat leak when not operating. However, the heat gain to the system is substantial due to the heat leak to the sump and pump, even when not filled with liquid; and due to both the sump""s and the pump""s thermal masses, and the consequent warming of the liquid when it is desired to return the sump and pump to the proper operating temperatures.
U.S. Pat. No. 5,860,798 issued Jan. 19, 1999 to Tschopp is representative of a more common type of cryogenic pump having both spring loaded inlet and outlet valves. The pump is located below its supply container and two connections to the supply container allow liquid to flow down to the pump and vapor to flow back, due to gravity. However, this type of pump cannot pump from a liquid source that is lower than itself, and is not satisfactory at very low NPSH conditions.
U.S. Pat. No. 3,430,576 issued Mar. 4, 1969 to the present inventor is for a low NPSH liquefied gas (liquid carbon dioxide) pump having a spring loaded inlet valve, but creates a temporary increase in suction pressure (NPSH) at the inlet valve during the intake stroke, so as to temporarily provide sufficient NPSH to open the spring loaded inlet valve. Variations of this are also found in the ""626, the ""940, and the ""798 patents. All require that the liquid be supplied to the pump.
U.S. Pat. No. 5,593,288 issued Jan. 14, 1997 to Kikutani is a liquefied gas pump shown in a mobile LNG application, top mounted and submerged to the bottom of the storage vessel; having a leakage path during the initial phase of the compression stroke back to the storage tank intended to allow vapor (bubbles in the liquid) to escape the compression chamber during the initial phase of the compression stroke, and thus avoid cavitation. However, the amount of vapor or bubbles can vary due to a number of factors, and thus excessive bubbles (and liquid) or insufficient bubbles can be allowed to escape, interfering with desirable pump operation.
A container for a cryogenic liquid that is stationary can be referred to as a vessel, and one that is mobile can be referred to as a tank, and tanks are considered to be smaller than vessels, but these terms can be used interchangeably.
The definition of a cryogenic liquid as used herein is one found in xe2x80x9cCryogenic Engineeringxe2x80x9d by R. B. Scott, Van Nostrand Co. 1959 which is that it is a liquid whose critical temperature is below terrestrial temperatures, taken as minus 70xc2x0 F. Examples include nitrogen, oxygen, argon, methane, hydrogen and natural gas, when in the liquid condition.
The definition of a liquefied gas as used herein includes cryogens but also substances (gases) when stored under conditions where the gas is in the liquid phase and where the storage temperature is below the ambient conditions there/then present. It can be a saturated liquid if it is at both the saturation temperature and pressure; it can be a sub-cooled liquid if the temperature of the liquid is lower than the saturation temperature for the existing pressure; and can be a compressed liquid if the pressure is greater than the saturation pressure for the temperature it is at. Examples include carbon dioxide, ammonia, and other low temperature refrigerants.
The present invention provides a system and method for reliably pumping cryogenic liquids to low, medium or high pressures when the pump is located even with, above or remote from, its source of liquid and the liquid may or may not be saturated. The low NPSH pumping system and method of the present invention provide the suction lift required to bring a saturated liquid (but also less demanding condition liquid) to the pump, and are equally capable of pumping under zero, very low, medium or high NPSH conditions or conditions where the NPSH varies during pumping. The system and method also are able to remove any vapor created by the pumping action, thereby preventing vapor locking or damaging cavitation of the pump. The system and method also offers a number of desirable options for utilizing the removed vapor. As such, they provide the unique versatility necessary to meet the varying conditions encountered in many pumping applications. Depending upon the needs of the entire system the pump is part of, the vapor can be returned to the source container, either below or above the liquid level in that container, or supplied to a vapor using need external to the tank or vessel (such as NG to an engine or other need).
One key element of this system is recognition that the amount of vapor encountered when bringing such liquids to the pump and filling the compression chamber of the pump with a cryogenic liquid or liquefied gas can vary greatly (either increase or decrease). This variation can result from a number of causes, even while pumping, as they are a function of many factors, some of which are: condition or available NPSH of the inlet liquid resulting from storage or flow characteristics of the inlet conduit or other reason, and incoming liquid vaporizing upon contact with warmed pumping chamber elements, the result of frictions. Another factor is residual liquid in the clearance volume expanding to vapor upon the depressurization accompanying the suction stroke as a result of the heat of compression (greater at higher discharge pressures), all resulting in vapor in the inlet side of the pump, which needs to be removed in order to effect reliable high pressure pumping of saturated cryogenic liquids.
Another key element is purposeful vapor removal from a saturated cryogen or liquefied gas located within the inlet conduit of a pump so as to provide suction lift, by causing the remaining cryogen or liquefied gas in the conduit to be cooled by evaporative cooling, thereby providing the differential pressure required for the suction liquid lift for a pump located above, alongside or remote from, the liquid source; and to essentially empty the liquid source. This process is progressive as the liquid in the inlet conduit continues to rise, and also is progressive as new liquid enters the inlet conduit. The cooled cryogen or liquefied gas can continue to be cooled and provided with lift so long as vapor is removed and the resultant evaporative cooling occurs faster than any warming of the evaporative cooled liquid in the conduit. The volume increase occurring when many of these liquids become gas is typically greater than about 40 to 1 under normal storage conditions, so a relatively small volume of liquid becoming vapor can result in a great volume of vapor. While it varies some for each liquid and storage conditions, a lift of over about 10 ft. for some saturated cryogens can result in a greater volume of vapor than liquid.
Accordingly, a pump system is provided that is able to satisfactory function under a wide variety of conditions; instead of the opposite situation, where the conditions must be correct for the pump to operate properly. This eliminates many special conditions and limitations faced in the past at pump installations for the cryogenic liquids and liquefied gases, especially the lower temperature cryogenic liquids. In addition, the dual compressing/pumping nature of this pump uniquely satisfies the requirements of mobile LNG fuel supply for dual injection pressure Diesel type engines and also has the capability to extend the storage life of the on-board LNG storage.
It should be understood that while the invention is described as especially useful for certain LNG applications, there are many other pumping applications involving LNG and other cryogenic liquids or liquefied gases where the dual path arrangement for supplying and pumping liquid and removing vapor from the intake side of the pump and the intake liquid container and other elements of the invention would find valuable use.